The present invention relates to apparatus for detecting objects, and more particularly to apparatus for detecting the presence or absence of objects by sensing reflected, disturbed or interrupted electromagnetic energy.
Heretofore, a transmitter emitted pulses of electromagnetic energy, which were reflected off an object. A receiver detected the reflected electromagnetic energy and determined the distance of the object by measuring the time interval between the transmission of the initial electromagnetic energy and the receiving of the reflected electromagnetic energy. The direction of the object was determined by the antenna location at the time of the transmission of the electromagnetic signal and at the time of receiving the electromagnetic signal.
In radar systems of this type, the receiver detected reflected pulse signals between transmitted pulse signals. As the range of the object was reduced, the time interval between transmitted and received pulses was shorter, which was more difficult to be observed by a operator. The reduction of pulse intervals required a shorter pulse width. Thus, pulse radar systems were not practical for use at close distances.
Radar systems were rather complex and expensive to manufacture. Such systems required large bandwidths for the intermediate frequency amplifiers to pass the pulsed signals and a local oscillator. Additionally, range gating magnetron and/or klystron tubes were required, which also were rather expensive. It was difficult to achieve large system gains and small bandwidths for the receiver.
Generally, the radar transmitter and the radar receiver employed a common antenna. As a consequence thereof, the scope of the directions covered was a function of the antenna beam width and antenna pointing position. To cover a wide scope of directions or segments of directions the antenna was rotated.
Doppler radar systems had transmitted electromagnetic signals continuously of a selected frequency, which signals were reflected from an object. A receiver detected the electromagnetic signal and recorded the frequency of the reflected electromagnetic signal. According to the Doppler effect, an object in motion will cause a variation in the frequency so that the reflected electromagnetic signal was different in frequency from the transmitted electromagnetic signal. Thus, the frequency of the reflected signal detected by the receiver was different from the frequency transmitted when the object was in motion. Thus, detection under this radar system required an object in motion.
The Doppler radar system required a sampling of radio frequency signals for a mixer action. Therefore, the transmitter and the receiver were generally at the same location and used a common antenna. Hence, the scope of the direction covered was a function of the antenna beam width and antenna pointing system. To cover a wide scope of direction or segments of directions, the antenna was rotated. In the Doppler radar system, it was the object that must be in motion. A moving antenna produced false readings. Excessive vibrating motion imparted to the Doppler radar system resulted in false readings.
When a transmitter and a receiver of a radar system were in close proximity to one another, a common antenna was generally used. Spatial coverage was then limited to the direction the antenna was pointed or can be pointed and the beam width of the antenna.
Additionally, field disturbance sensors have been heretofore employed, which were microwave intrusion sensors and devices that use R.F. energy for production line counting and sensing. Such field disturbance sensors have been pulse operated.